1. Field of the Invention
This invention relates to a recording apparatus which includes a process of forming electrostatic latent images on a charged photosensitive body by scanning with, for instance, laser beams and developing the electrostatic latent images. More particularly, the invention relates to a recording apparatus which is capable of recording multi-colored information on the photosensitive body with a plurality of laser beams.
2. Description of the Related Art
Recently, so called multi-color image laser beam printers have been developed which have a plurality of printing processes using scanning exposure, for instance with laser beams, and electrophotographic processes. This type of multi-color image laser beam printer is provided, for example, with a drum-shaped photosensitive body 20, as shown in FIG. 1. Around the circumference of photosensitive body 20 are arranged in sequence a first charger 21, a first exposure section 22, a first developing unit 23, a second charger 24, a second exposure section 25, a second developing unit 26, a transfer charger 27, a separation charger 28, a cleaner 29 and a discharging lamp 30. One process is completed by the following operations. Photosensitive body 20 is evenly charged by first charger 21. A first electrostatic latent image is formed at first exposure section 22. The first color is made visible by first developing unit 23. Photosensitive body 20 is recharged by second charger 24. A second electrostatic latent image is formed at second exposure section 25. The second color is made visible by second developing unit 26. Although it is not shown, if required, a control process is performed to make the charge polarities of the two color toners uniform. A two-color visible image is transferred onto a paper sheet 31 by transfer charger 27. After transferring, any toner remaining on photosensitive body 20 is cleaned off by cleaner 29. The remaining electrostatic latent image on photosenisitive body 20 is eliminated by discharging lamp 30.
However, the surface potentials of charged photosensitive bodies generally vary depending on individual differences between photosensitive bodies, which become fatigued due to continuous printing and temperature variations. Therefore, even in the past, charging potential control has been carried out through feedback control in order to eliminate the effect of these surface potential fluctuations. The feedback control is carried out by measuring the surface potential of the photosensitive body after charging using surface potential sensors. Nevertheless, since the installation positions of the surface potential sensors and the developing positions differ, even if the surface potentials at the positions of the surface potential sensors are maintained at constant values by the feedback control, the surface potentials at the developing positions will differ due to the dark decay of the photosensitive body. Dark decay is a phenomenon in which the surface potential of the photosensitive body decreases with passage of time.
Therefore, in the past, to compensate for the dark decay of the photosensitive body due to the differences between the positions of the surface potential sensors and the developing positions, the dark decay on the photosensitive body was measured in advance and the charging potential control constant was varied according to these results. However, by this method, the dark decay of the photosensitive body had to be measured beforehand, and it was difficult to obtain an accurate correction value for individual differences of photosensitive bodies. Therefore, there is still a problem in maintaining constant surface potentials at the developing positions.
In the prior art, as shown in FIG. 6, the surface potential is measured with a surface potential sensor located between the position of the charger and the position of the developing unit. The surface potential varies greatly due to the difference of dark decay between the charging position and the developing position. However, by carrying out this type of measurement, the fluctuation range of the surface potential is smaller due to proximity of the sensor to the developing position.
In the above method, it is possible to decrease the fluctuation of the surface potential. However, particularly in photosensitive bodies with large temperature variations or fatigue due to large continuous printing, perfect correction is difficult. In this case, a second control method can be considered.
By estimating the fluctuation component from the characteristics of the photosensitive body and changing the convergence value of the potential at the position of the surface potential sensor in advance according to the conditions, the difference from the actual the surface potential at the developing position is even smaller.
FIG. 7 shows another prior art method of controlling the surface potential for the case of a photosensitive body which has a slow dark decay at low temperature and a faster dark decay at high temperature. In this method, the potential is controlled at the developing position by setting the surface potential at the surface potential sensor position to be low at low temperature and high at high temperature. The situation is similar for fatigue of the photosensitive body due to continuous printing, so that the potential at the surface potential sensor position should be controlled by estimating beforehand the variation of the dark decay during continuous printing.
These situation may be summarized that, if the time during which the photosensitive body moves between the surface potential sensor position and the developing position is taken at T, the dark decay .DELTA.V at time T varies due to the temperature conditions and the continuous printing conditions, and if the required potential at the developing position is taken as V, the potential at the surface potential sensor position is given by EQU V+.DELTA.V
Here, when correcting for the temperature variation, the temperature in the vicinity of the photosensitive body is detected by a temperature detecting element and the correction can be effected by changing the value of .DELTA.V automatically. In the case of correcting for the continuous printing variation, the number of sheets printed is counted and the correction can be carried out by changing the value of .DELTA.V automatically.